1. Field of Invention
The present invention relates generally to a system and method for transfer of cryogenic fluids at a loading/unloading terminal. Specifically, the present invention provides a circulation loop within a single large size and inclined pipe to keep the system at/near cryogenic temperatures during idle periods between two consecutive transfer operations.
2. Description of the Related Art
Demand for natural gas is rising in developed countries but more natural gas is found in remote areas. To connect gas consumers and resources, one of the economically feasible methods is to liquefy the natural gas at −163° C. and then to transport the liquefied natural gas (LNG) with LNG tankers. In this LNG chain, LNG needs to be loaded into LNG tankers from onshore liquefaction plants at loading terminals near production fields and unloaded into onshore LNG storage at receiving terminals near consumers. The onshore storage tank is maintained at −163° C. and at a regular pressure (e.g. no more than 1.2 atmosphere pressure). Typically, a LNG tank is a full-containment type with vapor contained by outer concrete cylinder and roof LNG is stored in the inner tank typically made of stainless steel or nickel steel to a certain level leaving the space above for boil-off gas (i.e. vapor). Any vapor generated due to heat leakage through the tank wall/roof and transfer pipeline system flows into a gas pipe with intake level located above the maximum liquid level of the storage tank. The vapor is then compressed and typically sent to re-condenser.
LNG tankers are sea-going vessels and require a water depth of 15 m or more for docking and moving. Therefore, transferring LNG between onshore facilities and LNG tankers require a transfer system in a length varying from hundreds to thousands of meters depending on the seabed profile. Above-water pipeline systems for cryogenic fluid transfer at loading/receiving terminals are widely used along with supporting structures such as jetty or trestle. Under water systems are also proposed but only a few are built (e.g. transfer pipes located in an underground tunnel at the Cove Point receiving terminal in Maryland).
Typically, un-loading a LNG tanker takes about 12 hours and un-loading frequency is around twice per week. In order to avoid repeatedly heating/cooling which leads to early failure of the transfer system, it is required to keep the system at cryogenic temperatures during those internals between transfer operations. A conventional method is to have a separated LNG line (i.e., a small size recirculation line) that runs from the onshore storage tank to an offshore un-loading deck. During idle periods at a receiving terminal, a small amount of LNG is diverted into the recirculation line from LNG discharge pipe of the storage tank. This LNG flows to an offshore loading header and returns to the storage tank through the main transfer pipe. Boil-off gas appears after pressurized LNG is released from the circulating system into the tank in low pressure (e.g. 1 bar). This recirculation line may also be designed at an identical size as the transfer pipe, and both pipes serve as transfer lines during unloading operation.
In the conventional transfer system, both transfer pipe and recirculation line have their own insulation and water-proof outer shield. In the attempt to improve the system, U.S. Pat. No. 6,012,292 to Gulati and Silverman disclosures a system in which a small size recirculation line is positioned inside a transfer pipe. During idle periods, the cryogenic fluid is circulating through the recirculation line and the annulus between the transfer pipe and recirculation line. U.S. Pat. No. 6,244,053 to Gulati et al disclosures a system and method in which the transfer system is circulating single phase LNG at high pressure within a closed piping loop along with a heat exchanger for heat reduction.